1. Field of the Invention
This invention relates to improvements in the process for preparing alkyl perfluoro (4,7-dioxa-5-methyl-8-fluoroformylnonanoate)s.
2. Prior Art
Alkyl perfluoro(4,7-dioxa-5-methyl-8-fluoroformylnonanoate)s, ##STR1## where R is alkyl of 1-6 carbon atoms, are known compounds. They are used to make, e.g., by pyrolysis, the vinyl esters ##STR2## which have been copolymerized with other fluorinated olefins such as tetrafluoroethylene to provide copolymers which can be fabricated into membranes useful for separating the anode and cathode compartments of a chloralkali cell.
A process for preparing compounds of the formula ##STR3## where R is alkyl of 1-6 carbon atoms and n is 1-6 is disclosed and claimed in U.S. Pat. No. 4,131,740. In that process, a compound of the formula is contacted with hexafluoropropylene oxide (HFPO), preferably in the presence of fluoride ion as catalyst. It is disclosed that the fluoride catalyst is usually used in conjunction with an inert liquid diluent (preferably an organic liquid) in which the selected fluoride is at least 0.001% soluble, and that suitable diluents include ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and aprotic solvents such as acetonitrile. In Examples 2 and 3 thereof, ##STR4## was reacted with hexafluoropropylene oxide (HFPO) in the presence of fluoride catalyst and tetraglyme as diluent, to make ##STR5## which can be further reacted with HFPO as shown in Example 6. In Examples 4 and 5 thereof, V was reacted with HFPO in the presence of cesium fluoride catalyst and tetraglyme as diluent to make a mixture of products III (R.dbd.CH.sub.3), n=1-6, including both VI and ##STR6## Yields (i.e., yields corrected when possible by allowing for the amount of starting material recovered) to useable compounds VI and VII for these reactions, and E/O ratios (refer to the paragraph preceding the Examples below for the definition of E/O ratio) are given in Table I.
TABLE I ______________________________________ Yield (%) Based Yield (%) on HFPO Based on V to to to to E/O Example VI VII VI VII Ratio ______________________________________ 3 12.9 21.1 4 4.3 7.2 9.1 7.5 2.53 5 23.7 14.3 51.5 15.6 0.71 ______________________________________
In Example 6 thereof, VI was reacted with HFPO in the presence of cesium fluoride and tetraglyme to make VII and compound III (R.dbd.CH.sub.3), n=3; the yield of VII based on VI was 11% and based on HFPO was 20.5%, and the E/O ratio was 0.36.
It is clear that the process described in U.S. Pat. No. 4,131,740 gives a mixture of products III wherein different numbers of moles of hexafluoropropylene oxide have been added to the reactant IV. However, when a specific product is desired, e.g., III where n is 2 (i.e., compound I), loss of reactants to concurrent production of substantial amounts of other products, e.g., III where n is 1 and 3-6, is undesirable. Because the reactants IV and HFPO are costly to make, reactant IV being particularly so, loss of starting materials to other products when only compound I is desired incurs substantial loss of valuable precursor compounds.
Accordingly, it is an object of this invention to provide an improved process for making compound I in substantially higher yield than heretofore possible.